1. Field of the Invention
The invention relates to a method and a circuit arrangement for determining the entry and/or exit of a vehicle, in particular a traffic vehicle, into and out of a predetermined monitoring zone, comprising at least one inductive measuring loop which determines the monitoring zone and whose change in inductance, which results from the entry of a vehicle into the monitoring zone or the exit of a vehicle out of the monitoring zone, is used to control the oscillation frequency of an oscillator circuit, and more particularly wherein a measuring signal which corresponds to the relevant oscillation frequency of the oscillation circuit and a reference signal which corresponds to a reference oscillation frequency are subtracted from one another, and the established difference signal is processed in an analysis device for display purposes.
2. Description of the Prior Art
A method and a circuit arrangement of the above described type are already known in the art from U.S. Pat. No. 3,205,352. In this known method and in the known circuit arrangement which operates in accordance with this method, the oscillation signals emitted from a reference oscillator and from a loop oscillator which is connected to the inductive measuring loop are fed to a difference frequency detector whose output is connected to an analysis device whose components include a frequency selective amplifier and a detector. A disadvantage of this known method and of the corresponding known circuit arrangement is that, overall, only a relatively poor measuring accuracy is achieved at a viable expense. Moreover, it is occasionally considered disadvantageous that, in practice, in the known circuit arrangement only analog circuit elements can be used.
However, circuit arrangements constructed with digital-operating circuit elements are also known for determining the entry and/or exit of vehicles into and out of a predetermined monitoring zone for example U.S. Pat. No. 3,875,555. In this process, the loop oscillator which is connected to an inductive measuring loop is followed by a digital counter, and the reference oscillator is likewise followed by a digital counter. The counts reached by these counters are subjected to difference formation, and the difference signal thus formed is checked in an analysis device which contains a threshold value. In order to establish determinate measuring times, the two digital counters are supplied at special resetting inputs with control pulses which determine the times at which counting processes are initiated.
In order to achieve a higher measuring accuracy in this respect than the known circuit arrangement considered in the introduction, it is necessary to use digital-operating circuit elements with a high operating frequency at least in the control circuit zone which controls the operating sequence of the counters and of the analysis device. However, this involves the disadvantage of a relatively high circuitry expense. In other words, this means that the known circuit arrangements referred to above are not readily suitable to be constructed from electronic components which possess a relatively low operating frequency.
A digital loop detector system is also known from U.S. Pat. No. 3,868,626 in which the oscillation signals emitted from a loop oscillator which is connected to an inductive measuring loop are used to control a gate circuit which at its input is supplied with pulses emitted from a quartz-stabilised pulse generator and which at its output end is connected to an accumulator counter and also to a reference counter. The reference counter and the accumulator counter are connected by their outputs to a comparator which compares the count of the reference counter with that of the accumulator counter. The accumulator counter always contains the reference count determined by the preceding comparison. Therefore, in practice, the comparator compares a "current" count of the accumulator counter with an "old" count of the reference counter. When a stipulated threshold value is exceeded by the count difference thus established, a corresponding output signal is emitted. This known system also involves the disadvantage that, in order to achieve as high as possible a measuring accuracy, it is necessary to use circuit elements with a high operating frequency at least in the control circuit section. Furthermore, another disadvantage of the known system is that the slow changing frequency of the loop oscillator results in relatively small differences in the counts of the accumulator counter and the reference counter.
Finally, an arrangement is also known from U.S. Pat. No. 3,989,932 for establishing the presence of a vehicle in a zone of a line loop in which the line loop is connected to an oscillator which oscillates at a frequency which is dependent upon the inductance of the line loop. In this known arrangement a time clock circuit is provided which counts the duration of a predetermined number of periods of the oscillator signal. A reference signal stage is also provided which supplies a signal corresponding to a reference duration. A comparator stage determines the difference between the measured duration and the reference duration. Furthermore, the known arrangement comprises an analysis device having a threshold value stage which responds to a difference signal which exceeds a threshold value, and which produces a signal which indicates the presence of a vehicle in the predetermined zone of the line loop. Fundamentally, the construction of this known arrangement is identical to that of the known circuit arrangement considered in the introduction, although in contrast to the that circuit arrangement, in which frequency values are produced and related to one another, in the last-considered known arrangement time values are produced and related to one another. Although the last considered known arrangement operates on a digital basis, it also involves the disadvantage that, in order to achieve an adequate measuring accuracy, it is necessary to use circuit elements which have a relatively high operating frequency. This applies in particular to the control circuit which controls the time clock circuit and the reference signal stage.